As a modulation-demodulation method used in an optical communication system, a DPSK modulation-demodulation method is a communication method excellent in reception sensitivity. Accordingly, it is in particular expected for use in a long-distance optical communication system (see e.g. Literature 2).
Structure and operation of a common optical communication system using a DPSK modulation-demodulation method will be described with reference to FIG. 14 and FIG. 15. The optical communication system shown in FIG. 14 includes a transmission device 1301 for sending out a super-high speed optical signal of R (R: the number on the order of G (gigabit)) bps (bit per second), a reception device 1302 for receiving an Rbps optical signal transmitted over a long distance and an optical fiber 103 for transmitting an optical signal between these devices.
The transmission device 1301 comprises an N:1 multiplexer 1141, a DPSK modulation coding unit 1322 for receiving input of an Rbps multiplexing signal 1321 from the N:1 multiplexer 1141 to execute coding, and an electric-to-optical signal converter (electric-phase modulation optical signal converter) 1143. The reception device 1302 comprises an Rbps 1-bit delay interferometer 1332, an optical-electric signal converter 1334 and a 1:4 demultiplexer 1336.
FIG. 15 is a timing chart showing one example of an input/output signal string in the optical communication system shown in FIG. 14. The N:1 multiplexer 1141, with a number N of R/N bps signals D1˜DN synchronizing with each other as an input, multiplexes the signals bit by bit to output an Rbps output signal D. The DPSK modulation coding unit 1322, with the output signal D of the N:1 multiplexer 1141 as an input, executes coding for DPSK modulation to output a coded Rbps signal (electric signal) F. The electric signal-phase modulation optical signal converter 1143, with the Rbps electric signal F as an input, outputs an Rbps optical signal Q having phase information. In FIG. 15, Dij, Fij and D′ij represent j-th signals (indicative of data) in signals Di (i=1, 2, . . . , N), Fi and D′i.
The 1-bit delay interferometer 1332, with an Rbps phase modulation optical signal Q′ which has been transmitted through the optical fiber 103 as an input, detects a phase difference with a one-bit preceding signal to output an intensity modulation optical signal F′ corresponding to the phase deference. The optical-electric signal converter 1334, with the Rbps intensity modulation optical signal F′ as an input, outputs an Rbps electric signal D′.
Disclosed in Literature 1 is a method, related to duobinary modulation, of coding each of a number N of low-speed signals on the transmission side to multiplex the obtained signals on a time division basis after coding. The literature fails to recite a DPSK demodulation method on the reception side.    Literature 1: Japanese Patent Laying-Open No. 11-122205 (FIG. 1, paragraphs 0033˜0038, paragraph 0058).    Literature 2: Christian Rasmussen et al., DWDM40G Transmission Over Trans-Pacific Distance (10000 km) Using CSRZ-DPSK, Enhanced FEC, and All-Raman-Amplified 100-km Ultra Wave Fiber Spans, Journal of Lightwave Technology, U.S.A., January 2004, vol. 22, No. 4, pp. 203-207.
Optical communication systems in related art using a DPSK modulation-demodulation method have the following problems.
First problem is that a DPSK modulation-demodulation method makes a transmission device a high cost device requiring large power consumption. The reason is that a DPSK modulation coding unit operable at a super-high speed which is used in DPSK modulation is designed to flow a large amount of current to a transistor so as to bring out the best of performance by using high-cost advanced semiconductor process technology. In particular, application of a DPSK modulation coding unit operable at a super-high speed is directed to long-distance transmission for which mass production is not expected, so that cost-down by mass production effect is not expected either.
Second problem is that a system structure is not suitable for down-sizing of a device. The reason is that in the structure shown in FIG. 14, for example, a super-high speed signal is handled as an input/output signal of the DPSK modulation coding unit 1322. The super-high speed signal requires a high-quality connector for connection and a cable for connection in order to prevent signal quality degradation from causing a signal error. Therefore, space is required for layout to hinder compact packaging.
Third problem is that with the structure shown in FIG. 14, for example, co-existence of systems whose speeds are different makes the entire device expensive. The reason is that when a 10 Gbps system and a 40 Gbps system coexist, for example, a coding unit required for DPSK modulation-demodulation can not be commonly used and a 1-bit delay unit can not be shared for 10 Gbps and 40 Gbps either and they need to be prepared for 10 Gbps and 40 Gbps separately.
Under these circumstances, an object of the present invention is to provide low-cost and small-sized optical communication device and optical communication system using DPSK modulation requiring low power consumption.
Another object of the present invention is to facilitate improvement in speed from a 10 Gbps system to a 40 Gbps system and provide an optical communication device and an optical communication system whose costs are low and whose sizes are small as a whole even when systems with two speeds coexist.